Actinic keratosis (solar keratosis)
A precancerous lesion caused by cumulative sun exposure. It appears on sun-exposed areas of elderly individuals and in immunosuppressed patients (for example, organ transplant recipients). The lesions are usually red, have a rough surface, and show scaling. There is a 5%–20% risk of progression to squamous cell carcinoma. Treatment includes cryotherapy, topical application of fluorouracil cream, or surgical excision under local anesthesia.

Bowen’s disease
A precancerous condition that occurs mainly on the lower limbs of elderly individuals. The lesion appears reddish or pink and shows scaling. If left untreated, there is about a 10% risk of progression to squamous cell carcinoma, and therefore surgical removal is recommended.

Keratoacanthoma
A benign lesion that closely resembles squamous cell carcinoma and may regress spontaneously. It usually appears on sun-exposed areas of the body as a rapidly growing nodule with a keratin-filled central core. It can reach up to 2 cm within 1–2 months and typically regresses within six months. Because of the risk of malignant transformation into squamous cell carcinoma, surgical excision is recommended.

Sebaceous nevus
A precancerous lesion that appears at a very young age on the scalp as a yellow-orange plaque. After puberty, there is a 20%–30% risk of transformation into basal cell carcinoma. It is surgically removed, usually under local anesthesia, in one or more stages before puberty.